Add paren parsing

Grammar

@@ -11,6 +11,7 @@
 <expr> := if <expr> then <statements> end
        |  if <expr> then <statements> else <statements> end
        |  while <expr> SEP <statements> end
+       |  ( <expr> )
        |  <simple_expr>
 
 <simple_expr> := <id>

src/parser.rs

@@ -130,6 +130,12 @@ fn expression(tokens: &mut Tokens) -> ParseResult {
         Some(&Keyword(Kw::While)) => {
             while_expression(tokens)
         },
+        Some(&LParen) => {
+            tokens.next();
+            let expr = expression(tokens);
+            expect!(RParen, tokens);
+            expr
+        },
         _ => simple_expression(tokens)
     }
 }